Aromaticity and Relative Reactivity

The existence of these differential polarizabilities means (as indeed it does in benzenoid compounds) that a single cr+ parameter will not accu- 

The existence of polarizability thus renders impossible any unique scale of electrophilic reactivity. The two most common theoretical measures, namely ir-electron densities and localization energies, correspond to transition states approximating the ground state and the Wheland intermediate, respectively, whereas the transition state (the precise structure of which is unknown), lies somewhere in between, it Densities, which relate to a situation where inductive effects are dominant, will tend to predict a relatively low 2- 3-rate ratio since all of the heteroatoms are inductive acceptors (-/). By contrast, since it electrons are delocalized from the heteroatoms more to the 2- than to the 3-position, localization energies will predict a high 2- 3-rate ratio. The importance of these factors becomes particularly evident in consideration of the substitution of benzo derivatives of these molecules (Chapter 8). 

Additional problems in theoretical calculations are (1) selection of the coulomb and resonance integral parameters (2) whether an auxiliary inductive parameter be used for the a-carbons (3) whether d orbitals, etc., be taken into account for S, Se, and Te and (4) what type of calculations to use. 

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